In this paper we study the impact of query term expansion (QTE) using synonyms on patent document retrieval. We use an automatically generated lexical database from USPTO query logs, called PatNet, which provides synonyms and equivalents for a query term. Our experiments on the CLEF-IP 2010 benchmark dataset show that automatic query expansion using PatNet tends to decrease or only slightly improve the retrieval effectiveness, with no significant improvement. An analysis of the retrieval results shows that PatNet does not have generally a negative effect on the retrieval effectiveness. Recall is drastically improved for query topics, where the baseline queries achieve, on average, only low recall values. But we have not detected any commonality that allows us to characterize these queries. So we recommend using PatNet for semi-automatic QTE in Boolean retrieval, where expanding query terms with synonyms and equivalents with the aim of expanding the query scope is a common practice.